Why would you size for multiple incidents? One is rare enough, but you have to deal with it. Two guarantees a whole set of people and equipment that will never have a use.
Anyway, I suspect the real reason is that an emergency is a dynamic situation, and an airport is a large complex barely-controlled environment. Sure, probably parts of the airport could stay in use, but it would take a lot of coordination and analysis to do so safely, and there's really no time to do that. So you just shut it down.
You know what's going on now, so it's easy to say; but if you're running the airport and all you know is "aircraft on fire", what do you do?
Anyway, I suspect the real reason is that an emergency is a dynamic situation, and an airport is a large complex barely-controlled environment. Sure, probably parts of the airport could stay in use, but it would take a lot of coordination and analysis to do so safely, and there's really no time to do that. So you just shut it down.
You know what's going on now, so it's easy to say; but if you're running the airport and all you know is "aircraft on fire", what do you do?